Objective: To characterize the physical and chemical events leading to the expression of radiation-induced damage at the cellular level. Methods: The above objective has been approached mainly through the use of the spores of Bacillus megaterium as the test organism. Other systems have and will be used such as Phage T7 and Aspergillus (both used in the past) and vegetative cells and mammalian cells (planned for the future). Both X-rays and UV have been used. Results: The relationship to water H2O or D2O) and water structure to radiation sensitivity has been clarified through experiments in "dry" systems and damage in "dry" systems compartmentalized into several types whose characteristics have been defined. A proposed model or radiation sensitization in solution has been presented involving the OH radical and H2O2. The sensitizing action of the metal ions Ag, Cl and Fe are being studied biologically as well as chemically so as to understand the mechanisms involved in radiation sensitization. Mechanisms involving organic senstizers such as PNAP and diacetyl are under study. The effect of nitrate in radiation sensitization of the bacterial spore has been reported and the effects of nitrate concentration on radical yields chemically has been reported. Further studies on other metal ions are planned. These studies will involve biological end-points and related radiation chemical experiments using both pulse radiolysis and steady state irradiation.